Supplemental Lecture (97/01/10 update) by Stephen T. Abedon (abedon.1@osu.edu)

1. Chapter title: Water
1. A list of vocabulary words is found toward the end of this document
2. Life evolved in water and all metabolically active life either lives in water or carries water around with it, such that a high fraction of the make up of an organism (50%) usually consists simply of liquid water . Water is a chemically unique compound to which life is both fully and irreversibly adapted. Outside of the cell, nutrients are dissolved in water , which facilitates their passage through cell membranes. And inside the cell, water is the medium in which most chemical reactions take place.
3. Unusual and/or important properties of water include: high polarity, hydrogen bonding, cohesion, adhesion, high specific heat, and high heat of vaporization. water is, by far and away, the most common molecule that exists as a liquid at typical ambient Earth temperatures. Water is a powerful dissolver of polar ions and molecules (polar solvent), and a similarly powerful excluder of nonpolar molecules. These properties of water are all important and/or crucial to the existence of life as we know it.
0. Reduction
1. Water consists of reduced oxygen.
2. Reduction is the addition of electrons to an atom or a molecule. (Think of reduction as some kind of reduction--sort of--in charge since electrons hold a negative charge).
3. Addition of hydrogen atom:
1. Often reduction leads to no net loss of charge because protons may also be added. In fact, very often reduction occurs as a transfer of both an electron and a proton: a hydrogen atom.
2. For life, the most important example of reduction is the addition of a hydrogen atom (one electron and one proton) to an atom or molecule.
3. When you think hydrogen, think reduced.
4. See lecture on the energetics of life.
1. Oxidation
1. Water consists of oxidized hydrogen.
2. Oxidation is the removal of electrons from an atom or molecule.
3. Oxidation-reduction:
1. Note that when an atom or molecule is oxidized, another atom or molecule must similarly have been reduced (i.e., the atom that attracted the electrons).
2. For example, when hydrogen atom addition results in reduction, the hydrogen atom undergoes (by definition) oxidation since it is donating an electron.
4. Electronegativity:
1. Certain atoms, described as highly electronegative, tend to very strongly attract electrons.
2. One very important highly electronegative, and consequently highly oxidizing atom is oxygen (no big surprise here).
3. Oxygen atoms very strongly attract up to two electrons and hence up to two hydrogen atoms.
4. See lecture on the energetics of life.
2. Oxidized hydrogen/reduced oxygen
1. Chemically, water consists of fully oxidized hydrogen, or fully reduced oxygen. That is, in the formation of water from molecular hydrogen, H₂, and molecular oxygen, O₂, each hydrogen atom essentially partially donates its electron to oxygen without full reciprication.
3. Tetrahedron
1. The outer electron shell of a fully reduced oxygen atom (or carbon atom, etc.) consists of 4 pairs of electrons.
2. These electron pairs are arranged about the oxygen nucleus such that about pairs show maximal separation from other pairs. This is because their negative charges repel one another.
3. If you were to represent paired electrons as small balls and connect them by sticks to a not represented oxygen nucleus, you would recognize the shape as being that of the play toy, jacks. The angle formed by each pair of "sticks" is 109.5°. See figure below.
4. Illustration, tetrahedron



5. Modified tetrahedron
1. In water molecules the hydrogen atoms lie somewhat closer together than they would if arranged in an ideal tetrahedron (i.e., the angle between them is 104.5° rather than 109.5°).
6. Illustration, modified tetrahedron



7. Partial charges
1. Because hydrogen is such a good electron donor and oxygen such a good electron acceptor, in a water molecule each hydrogen atom tends to partially give without reciprocation its electrons to oxygen which, in turn, gladly accepts them.
2. As a consequence of this arrangement, the oxygen atom takes on a partial negative charge while the hydrogen atoms each take on a partial positive charge. Another way of putting this is that "each oxygen atom has seven protons more than either of its hydrogen partners, so the covalently shared pairs of electrons in a water molecule spend more time around their O than about either H (von Hippel, 1994)."
8. Illustration, water partial charges



9. Polar molecule
1. This taking on of a partial negative charge is typical for oxygen atoms covalently bonded to other, less oxidizing atoms.
2. The molecules thus formed are referred to as polar molecules.
3. Water, the prototypical polar molecule:
1. Water is the prototypical polar molecule. That is, water exhibits a partial positive charge at one end and a partial negative charge at the other.
2. Here water having ends is a consequence of the hydrogen atoms in water being arranged about the oxygen atom in a modified tetrahedron such that the hydrogen atoms are closer to each other than they otherwise would be. Thus, the close hydrogen atoms make up a positive end of a water molecule while the other end consists of the partially negative oxygen atom.
10. Hydrogen bond [H-bond]
1. A hydrogen bond is a non-covalent bond between a partial negative charge and a partial positive charge.
2. Hydrogen bonds tend to be weak. Covalent bonds are about 20 times stronger than a typical hydrogen bond.
3. Hydrogen bonds tend to be transient. In liquid water these hydrogen bonds between water molecules tend to be very transient (lasting only 10^-11 seconds!).
4. Hydrogen bonds are very numerous which somewhat offsets their weak and transient nature. Water molecules readily hydrogen bond between each other. On average each water molecule in liquid water is hydrogen bonded to 3.4 other water molecules.
5. Results in unusual water properties:
1. Hydrogen bonds are collectively responsible for a large number of water's properties, including:
1. being a liquid rather than a gas a room temperature
2. having a high specific heat
3. having a high heat of vaporization
4. serving as a powerful solvent of polar molecules
5. effecting adhesion
6. effecting cohesion
7. effecting hydrophobic exclusion
8. ice floats
11. Cohesion
1. The attraction of one water molecule to another resulting from hydrogen bonding.
2. By placing a drop of water on a surface you can directly observe cohesion in the resistance that water droplet shows to wetting, i.e., water clumps up in a pile despite being a liquid, rather than spreading out over the surface. (note that wetting less likely occurs in the absence of adhesion to the surface being wet.)
12. Water is a liquid at typical ambient temperatures
1. By molecular weight (MW), ought to be a gas:
1. CO₂ (MW=44), O₂ (MW=32), CO (MW=28), N₂ (MW=28), CH₄ (MW=18), and H₂ (MW=2) are all gasses at room temperature.
2. Water (MW=20) is a liquid. Why?
2. Because water molecules display cohesion and thus have a much reduced tendency to fly off into the overlying atmosphere than these other listed molecules.
13. High specific heat
1. By definition, a temperature increase is an increase in the motion of the molecules and atoms making up a substance.
2. Because of cohesion, water molecules resist increasing their motion. (this is another way of saying that is water molecules resist the net breaking of hydrogen bonds). Consequently, water resists heating; water has a very high specific heat.
3. This tendency to not want to change temperature both causes resistance to radical temperature swings within beings and causes bodies of water (e.g., a lake) to strongly resist rapid changes in temperature. This temperature buffering capacity of water is taken advantage of to a great extent by organisms.
14. High heat of vaporization [evaporation]
1. Because it also involves the breaking of hydrogen bonds, water resists vaporizing (evaporating). Consequently, it takes a lot of heat to evaporate water. This high heat of vaporization is also utilized by organisms as a cooling process, e.g., sweat or panting.
15. Adhesion
1. Adhesion is similar to cohesion except with adhesion involves the attraction of a water molecule to a non-water molecule. cohesion is thus a special case of adhesion.
16. Hydrophilic substance
1. Polar molecules and charged ions. In other words, hydrophylic substances are those to which water tends to adhere.
17. Water as a hydrophilic solvent [dissolve, solute, solution, solvent]
1. review of solvent, solute, and solution:
1. Solute: A substance that is dissolved into a solution. Most biological molecules are found in liquids (usually water) in which they have dissolved and therefore may be considered to be solutes.
2. Solvent: A substance in which a solute has or can dissolve. The most important solvent to life is water.
3. Solution: A medium consisting of solutes and solvent(s). Metabolic reactions occur in solutions, usually solutions in which water is the solvent.
4. Dissolve: The act of a solute going into solution.
2. Water tends to very efficiently dissolve hydrophilic substances (both polar molecules and those which ionize upon dissolving).This occurs as a consequence of adhesion.
18. Hydration shell:
1. Particularly, water molecules form a hydrogen bonded layer, called a hydration shell, that surrounds hydrophilic substances/A. This shell adheres so powerfully that it is actually more energetically favorable for many polar substances to exist as individual molecules surrounded by hydration shells than to remain within a homogeneous solid material. Thus, hydrophilic substances/A tend to dissolve in water.
2. Any condition that increases the rate at which water can form a hydration shell (heating, stirring, shaking, adding an excess of water), the faster a hydrophilic crystalline substance will dissolve.
19. Hydrophobic (non-polar) substance
1. An atom or molecule to which water does not readily adhere. Hydrophobic substances tend to not readily dissolve in water.
2. Many biological solutes do not like to dissolve in <EMwater, i.e., are hydrophobic. These molecules tend to clump together away from <EMwater (hydrophobic exclusion). In a sense, these solutes end up "dissolving" in a solvent consisting of other hydrophobic solutes.
3. See lipid.
20. Hydrophobic exclusion
1. Energetically unfavorable hydration shells:
1. When dissolved in water, an individual hydrophobic molecule is, by definition, maximally surrounded by the water molecules of its hydration shell.
2. However, because those water molecules of its hydration shell do not readily hydrogen bond to the hydrophobic molecule, the presence of a hydration shell is energetically unfavorable. (that is, the water molecules would much rather be free to hydrogen bond with one another).
2. Note that schematically one can see (below) that it takes more water molecules to surround two dissolved separate hydrophobic molecules than to surround two closely spaced hydrophobic molecules:
3. Hydrophobic exclusion:
1. This decline in total molecules required to surround non-polar molecules that have banded together leads to a phenomenon known as hydrophobic exclusion. You may know this phenomenon better as "water and oil don't mix."
2. As we shall see, the hydrophobic exclusion of biomolecules plays a crucial role in the physiology of life, as crucial as the dissolving of hydrophilic molecules.
21. Illustration, hydrophobic exclusion



22. Amphipathic (or amphiphilic, micelle)
1. A compound having both a hydrophilic and a hydrophobic end. For example, soaps and detergents. See diagram below depicting amphipathic molecules congregating in a water solution into a structure known as a micelle.
23. Illustration, micelle



24. Ice floats
1. The density of water is actually less than it could otherwise be because hydrogen bonded water is packed slightly less favorably than could be achieved without hydrogen bonding.
2. Ice represents a maximal hydrogen bonding of water, indeed the crystallization of water into the structure formed upon hydrogen bonding. Thus, ice occupies a greater volume per unit mass and, consequently, floats on water.
3. Of similar importance, high pressures tend to inhibit the solidification of water rather than enhance it. Thus, the bottom of oceans and lakes tend to remain in the liquid phase while the upper reaches tend to be the first to freeze.
25. Vocabulary
1. Adhesion
2. Amphipathic
3. Cohesion
4. Dissolve
5. Evaporation
6. Heat of vaporization
7. Hydration shell
8. Hydrogen bond
9. Hydrophilic
10. Hydrophobic
11. Hydrophobic exclusion
12. Ice floats
13. Nonpolar molecule
14. Oxidation
15. Polar molecule
16. Reduction
17. Solute
18. Solution
19. Solvent
20. Specific heat
26. Practice questions
1. Which bond (shown as a dash (-) or as an equal sign (=) between explicitely shown atoms) is more reduced? Assume that all molecules are otherwise identical (circle one correct answer) [PEEK]

		(a) |
		   -C-H
		    |

		(b) | 
		   -C=O

		(c) | 
		   -C-OH
		    |

		(d) |   |  
		   -C-O-C- (consider first C-O bond only)
		    |   |

		(e) all are equivalent

2. True or False, hydrogen bonding is a property that explains the existence of, and is unique to the process known as cohesion (circle best answer). [PEEK]
3. Hydrophobic exclusion occurs upon the addition of ___________ substances to water (circle best answer). [PEEK]
1. Polar
2. Ionic
3. readily hydrogen bonded
4. hydrophobic
5. all of the above
6. none of the above
4. What are nutrients dissolved in?[PEEK]
5. The existence of partial and opposite charges at either end of water molecules is responsible for water having (circle best answer): [PEEK]
1. a high heat of vaporization
2. hydrogen bonding between water molecules
3. cohesion
4. the existence of oceans
5. all of the above
6. none of the above
6. In terms of mass, what is the most abundant element in most organisms? (circle best answer; yes, this is readily derivable from what you know--though not necessarily straightforwardly so): [PEEK]
1. hydrogen (atomic mass=1)
2. carbon (atomic mass=12)
3. oxygen (atomic mass=16)
4. calcium (atomic mass=40)
5. iron (atomic mass=56)
6. none of the above
7. The existence of partial and opposite charges at either end of water molecules is responsible for (circle best answer): [PEEK]
1. supernova
2. water easily evaporating
3. cohesion between water molecules
4. the existence of water vapor
5. all of the above
6. none of the above
8. Name or draw a simple molecule (not ion) in which at least one of the atoms is as completely oxidized as you are likely to find in a biological system. [PEEK]
9. What kind of interaction between water molecules is responsible for the occurrence of cohesion? [PEEK]
1. ionic bonds
2. covalent bonds
3. van der Waals interactions
4. hydrogen bonds
5. all of the above
6. none of the above
10. Which of the following is not a consequence of water molecules displaying partial charges arranged on opposite ends of the molecule? [PEEK]
1. cohesion
2. adhesion (to other polar molecules)
3. high boiling temperature
4. liquid at room temperature
5. all of the above
6. none of the above
11. What chemical and/or physical aspects of water makes it so unique and important to life? (circle one correct answer) [PEEK]
1. small size and polarity.
2. large size and non-polarity.
3. refreshing taste.
4. low density.
5. all of the above (are unique and important aspects of water).
6. none of the above (are unique and important aspects of water).
12. Which of the following properties is not associated with the bonds between water molecules found in the liquid state? (circle one correct answer) [PEEK]
1. non-covalent.
2. weak.
3. transient.
4. intermolecular attraction between partial charges.
5. all of the above (are properties of these bonds).
6. none of the above (are properties of these bonds).
13. What property of water results in its being a liquid at typical ambient temperatures? (circle best answer) [PEEK]
1. low molecular weight.
2. lack of polarity.
3. adhesion.
4. cohesion.
5. all of the above.
6. none of the above.
14. What kind of bond attracts water molecules to other water molecules? [PEEK]
1. covalent bond.
2. polar covalent bond.
3. hydrogen bond.
4. ionic bond.
5. all of the above.
6. none of the above.
27. Practice question answers
1. a, remember to think reduced when you see a C-H bond.
2. False, hydrogen bonding occurs between molecules other than just water.
3. iv, hydrophobic
4. water
5. v, all of the above
6. iii, oxygen. The logic is as follows: Water makes up more than 1/2 of all organisms. Presumably this figure is by weight. Oxygen is 16/18 of water. Even if water made up only 1/2 of most organisms and hydrocarbons made up the rest (which they don't), then oxygen would still likely dominate the mass of organisms 16/18 to 12/14 (atomic mass oxygen / MW water vs. atomic mass carbon / MW CH₂).
7. iii, cohesion between water molecules
8. water or carbon dioxide
9. iv, hydrogen bonds
10. vi, none of the above
11. i, small size and polarity.
12. v, all of the above are properties of these bonds.
13. iv, cohesion.
14. iii, hydrogen bond.
28. References
1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 28-34.
2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 29, 33-34.
3. von Hippel, Arndt (1994). Human Evolutionary Biology. Stone Age Press, Anchorage, AK. pp. 31-33, 34-35.